Melt Butter: Place butter in a small saucepan and melt over low heat, stirring occasionally. Alternatively, melt it in the microwave in 30-second bursts. Once melted, pour the butter into a large mixing bowl and let it cool for 10-15 minutes until it's slightly warm to the touch, not hot.
Prepare Wet Mixture: To the cooled melted butter, add both sugars and whisk for 20 seconds. Then add the egg and vanilla, whisking until well combined, about 15 seconds.
Incorporate Dry Ingredients: Sift in the flour, cocoa, and baking soda. Add the salt. Mix with a spatula or wooden spoon until a thick dough forms.
Add Chocolate Chips: Gently fold in the chocolate chips.
Chill & Preheat Oven: Refrigerate the dough for 1 hour. With 30 minutes of chill time remaining, preheat your oven to 400ºF (200ºC). Line a large baking sheet with parchment paper and set aside.
Form Cookies: Scoop 45g / 1.5 oz (or 3 level tablespoons) of dough and roll into tall, cylindrical balls. Place them on the lined baking sheet, spaced 4 inches / 10cm apart. Do not flatten the balls.
Bake: Bake for 10-12 minutes until edges are set but centers are soft. Immediately after removing from the oven, place a few extra chocolate chips on top of each cookie so they melt slightly and turn shiny. Allow to cool on the sheet for 10 minutes before transferring to a wire rack to cool completely. Enjoy!